Oligochaeta

Oligochaeta is a subclass of soft-bodied annelid worms, nested within the class Clitellata, and comprises roughly half of all known annelid species, some 10,000 in total. The group includes the terrestrial megadrile earthworms, some of which are semiaquatic or fully aquatic, along with smaller freshwater and semiterrestrial microdrile forms such as tubificids, pot worms, ice worms, and various marine interstitial species. Oligochaetes are well segmented, have a spacious fluid-filled coelom that acts as a hydrostatic skeleton, and typically bear few setae, or bristles, on their outer body surface, and they lack the fleshy parapodia found in their relatives the polychaetes. Their size spans an extraordinary range, from worms under half a millimeter long to giant earthworm species reaching two to three meters.